No Send Inbox Option
ProtonMail is missing a feature that is a "no brainer" for most email accounts, including gmail.
If I have an email in "Sent" and I want to move it to my Inbox, there is no option. I can only move to a folder.
And then you can use that to move it to the inbox. It's definitely possible. The problem is that 4 action icons have to be selected to show under an email and this is predetermined by Proton. It'd be nice to be able to customize actions for emails for all folders as well as specific ones and then there could be one icon for "Move to inbox".
Otherwise, this will always piss off some users, because it will not suit their mental model of email management.